Hardness precursors such as barium, calcium, magnesium, iron, silica, carbonate and bi-carbonate, fluoride and sulfate are commonly found in surface water supplies, water wells and aquifers and in aqueous industrial effluents such as cooling tower blow-down. These sparingly soluble contaminants limit the percentage recovery of purified, desalinated permeate from reverse osmosis (RO) and nano-filtration (NF) membranes, as they tend to form scale compounds upon concentration, which deposit on the surface of the membranes and reduce their useful service life or require very frequent membrane cleaning frequencies.
A simple RO membrane system will typically achieve a maximum permeate recovery of approximately 70%. Conventional home type RO systems, also known as Whole House RO systems are typically operated with a permeate recovery of 25% to 50%. At these low permeate recoveries, however, 1-3 gallons of water will be wasted for every gallon of purified or desalinated drinking water that is produced. This low membrane permeate recovery is intended to completely mitigate the risk of build-up of fouling material including natural organic matter, colloidal matter and scale compounds, thereby minimizing maintenance and reducing the frequency of cleaning or replacement of the costly RO membranes. This low membrane permeate recovery also enables the designer to simplify the RO system design by eliminating use of pretreatment chemicals and membrane cleaning chemicals. These advantages are offset, however, by the poor performance of these basic home or “whole House” RO systems since they generate large volumes of wasted RO reject water and use much more influent (e.g. city water or well water) volume than the volume of the purified water actually consumed by the end-user. In this manner, any savings in the cost of the system are offset by the higher cost of influent water and the sewer discharge costs.
Current improvements in the design of these home RO systems include the use of ion exchange softening resins as pretreatment before the RO membranes in some “Whole House” RO systems, to remove the hardness from the influent water, thus enabling the RO membranes to achieve higher permeate recoveries. These improved systems achieve RO permeate recoveries of 60%-70%, but continue to waste 30%-40% of the influent water as RO rejects to drain. Furthermore, the water softener ion exchange resin must be periodically regenerated by using commercial (i.e. sodium chloride) salt solutions of up to 10% strength to ensure effective regeneration and rinsing of the IX resin and recovery of the resin capacity between regeneration cycles. Further still, fresh water or good quality RO permeate must be used to prepare the regenerating salt solution, thus further reducing the net purified water recovery by an additional 2-5%, while wasting a costly chemical product and adversely impacting the environment and human health by releasing sodium and chloride ions to receiving surface water.
In addition to the foregoing description, it is notable that previous efforts to develop suitable RO systems have been concerned with increasing the membrane process product water recovery and addressing the problems associated with use of commercial salt by using RO concentrates to regenerate the SAC IX resins used as pretreatment for the RO, but despite many efforts in this area, these processes remain inefficient. For instance, they have not been able to achieve a membrane system product water recovery in a highly desirable range (viz. >90% and even >95%), while simultaneously reusing the membrane concentrate to regenerate the IX resin, thereby further improving the net product water recovery and eliminating use of commercial salt for regenerating the IX resin. It is also known that the influent water quality, and in particular the total dissolved solids (TDS) and the concentration of hardness and other multivalent cations and silica associated with surface water and more particularly with groundwater, will vary over time, for example over a period of five years. Thus, previous work in the RO field do not address this variability in the influent water quality and the need to have reliable pre-membrane and post-membrane water softening capability to minimize the adverse impact of hardness and other colloidal multi-valent ions on the membrane permeate flux. The present invention meets these and other needs.